This invention relates to a system for remote detection, location, identification and monitoring of physical objects including persons and vehicles. This system includes a responder, such as a remote tag device, and an interrogator. This device is capable of accepting a signal, uniquely encoding the signal and retransmitting the encoded signal back to an interrogator.
Prior art interrogator-responder systems have employed a variety of methods to return the desired response to the interrogator. One method allowing unique responder identification by the interrogator uses matched transmitter-receiver sets, with each set tuned to a different frequency. Because the number of transmitter-receiver pairs increases with each object or person to be monitored, large scale systems of this type are complex, unwieldy and expensive.
Other systems use responders that broadcast unique identification signals for acquisition and recognition by the interrogator. However, prior art does not impose the desired responder information upon a retransmitted interrogator signal. Instead, the responder internally generates and transmits a signal with the proper characteristics. Time reference and synchronization means may be needed to create a desired identification code and to effect code transmission sequencing. Accordingly, such systems possess substantial power requirements.
Responders powered by the collected interrogator signal, as found in magnetic or inductive coupling systems, require close proximity or precise alignment between the responder and the interrogator for proper operation.
Responders typically transmit a broad, non-directional beacon signal and not a narrow, retrodirective beam. Generally, a non-directional beam demands more power to transmit a signal to the interrogator than does a highly directional beam at equivalent distances. Therefore, a low-power, non-directional broadcast from a responder is generally unsuitable for certain applications, e.g., in noisy environments or where large distances exist between interrogator an responder.
To form a coherent, retrodirective wavefront, prior art Van Atta arrays require adjusting the length and characteristic impedance of each transmission line which interconnect the paired antenna elements. Nagai, U.S. Pat. No. 3,731,313, teaches a linear Van Atta array which achieves retrodirectivity by inserting impedance matching stubs into the interconnecting transmission lines. Although the impedance matching stubs eliminate the need for adjusting the interconnecting transmission line length, stub length requires adjustment to achieve retrodirectivity.
Trenam, U.S. Pat. No. 3,938,151, teaches a Van Atta array of printed circuit radiators that does not require adjustable transmission lines or impedance matching stubs to achieve retrodirectivity. This array does not, however, modulate or transform the incident wavefront prior to reflection towards the direction of the source. In addition, Trenam's art consists of large, discrete components intended to cover the surface of a large decoy balloon.
Pittman, et al., U.S. Pat. No. 5,064,140, teach a two-dimensional Van Atta array which imposes an information-carrying modulation upon the collected wavefront before retrodirective transmission to the original source. This art also teaches amplification of the signal before retrodirective transmission.
Small, low-power, inexpensive responder devices are useful for applications such as vehicle tracking, runway and road marking, personnel identification, remote process monitoring and meter reading.
It is important for such devices to have the following capabilities:
(1) Respond to an interrogator only when illuminated by that interrogator;
(2) Respond to interrogations arriving from any direction within a large solid angle so that the orientation of the device is not critical;
(3) Confine the device's response to the interrogator to a small solid angle centered about the location of the interrogator;
(4) Operate through adverse weather conditions (rain, fog, etc.); and
(5) Provide a unique response which would permit the interrogator to distinguish the tag device from surrounding clutter return and other tag devices.
Short wavelength (e.g., microwave or millimeter wavelengths) operations are desirable because the directive interrogation can be achieved with an interrogator antenna of convenient size, and the narrow retrodirective beam can be formed by a physically small device. Because the retrodirective beam is concentrated within a narrow solid angle, interrogator power requirements are reduced.
Small device size facilitates the use of monolithic semiconductor materials which decreases the cost of production while increasing the reliability of the devices.
The prior art devices are not suitable for small, inexpensive and low-power applications or high-volume fabrication and production with the above capabilities. The device described herein differs from prior solutions in the following ways:
(1) The responder device or tag requires no radio-frequency sources or amplifiers, because the incoming interrogation signal is modulated by a low frequency source at the device before being transmitted back to the interrogator.
(2) Less radio-frequency interrogator power is required because the interrogator beam can be very directive and the evoked response is concentrated in a narrow beam in the direction of the interrogator.
(3) The device can be produced in large quantities using monolithic technology.